Enhancement: mount: Enforce FUSE Unix permissions by default

By default, `mount` will now respect the Unix permissions of the files within
snapshots (this is done through the "DefaultPermissions" FUSE option).

To restore the old behavior, we've added the `--no-default-permissions` option.
This allows alll users that have access to the mountpoint to access all
files within the snapshots. Normal FUSE rules apply, so `--allow-root`
or `--allow-other` can be used to allow users besides the mounting user to
access the mountpoint.

https://github.com/restic/restic/pull/2017